Industrial gases such as nitrogen and oxygen are produced commercially in large quantities by the cryogenic rectification of feed air. Refrigeration to drive the cryogenic rectification is provided by the turboexpansion of a compressed process stream which is generally either a compressed feed air stream or a high pressure waste stream taken from the rectification column. The turboexpander of an air separation plant is a costly piece of equipment to operate and maintain and it would be desirable to reduce such costs.
Helical screw compressors are inexpensive and durable. However, their use in reverse as expanders is not desirable because the oil flooded type would contaminate the process fluid and the independently geared non-lubricated rotor type is less efficient. Moreover, even the non-lubricated independently geared rotor type is susceptible to process fluid contamination from the bearing lubricant even though at high temperatures the rotor expansion would serve to improve efficiency. Still further, in cryogenic service the cold temperatures would freeze the lubricant and the rotor contraction would serve to further reduce efficiency.
It is an object of this invention to provide an improved helical dry screw expander which may be effectively employed in a cryogenic production cycle.
It is another object of this invention to identify a cryogenic production cycle which can effectively employ a helical dry screw expander without experiencing unexceptably high increased power costs.